Four weldments of varying levels of weld joint to baseplate strength (mismatch) were examined to determine the effects of this ratio on fracture performance. Cracks were placed within the weld metal at several ratios of weld fusion margin. This margin is defined as the ratio of crack tip proximity to the weld fusion line and crack length (Lcrk/a). Finite element studies were then performed on each variation of weld fusion margin and mismatch level to determine the crack driving force not accounted for in a typical fracture analysis, which assumes monolithic, homogenous weld metal specimens and ignores the weld/baseplate interaction. Assuming homogenous weld metal specimens results in crack growth resistance behavior that depends on crack tip proximity to the weld fusion line. Once local plasticity and the additional constraint caused by the weld/baseplate interface are included in the analysis, fracture behavior is independent of crack tip proximity to the fusion line up to and slightly beyond initiation. Errors in fracture toughness at weld fusion margins greater than or equal to 1.5 are less than 10% and can reasonably be ignored. Errors at smaller values of Lcrk/a are considerable, and effects of local plastic deformation and constraint must be accounted for in the fracture analysis. Furthermore, uncorrected estimates of initiation toughness (JIC) in specimens where the weld fusion margin are less than 1.5 result in overly conservative estimates. The percentage of weld mismatch was found not to be a major factor in determining either crack growth resistance or crack initiation behavior. An increased propensity toward unstable fracture was observed in specimens with smaller Lcrk/a ratios. This effect may be attributed to a local microstructural defect and requires further study.